Antisera against cell surface antigens of mouse T- and B-cell tumors are being raised in rabbits, in syngeneic and allogeneic mice and in rats. These cells represent a relatively homogeneous source of antigen in contrast to a normal lymphocyte population and should retain many premalignant antigens. These antisera will be absorbed by in vitro and in vivo procedures using syngeneic and allogeneic mice, both normal and those bearing different T- and B-cell tumors. These antiserum preparations should be cytotoxic for different populations of normal mouse lymphocytes. This, and the functional diversity which exists among T-lymphocytes in particular, should permit dissection of normal populations and antigenic recognition of different functional subpopulations. Thus, normal lymphocyte populations will be assayed for various T-cell functions, e.g., helper function, cell-mediated cytotoxicity and the graft-versus-host reaction, before and after treatment with a given antiserum preparation and complement. Some of the antigenic populations may be a relatively small portion of the total population and a related function be made more apparent by testing the survivors after killings with one or more other antisera. The antigens involved in this work are expected to be other than the established surface antigen, e.g., H-2, Ly, theta and TL. Among the sought-for antigens are antigen-binding receptors which, in the case of the presumed antigen-binding receptor on the cells of a given lymphoma, should be of a relative high concentration compared to the heterogeneous normal lymphocyte preparations frequently used. The recognition of such receptors is discussed. Other antigens, not now recognized because of dilution in normal cell preparations, may also be found. Antigens associated with a particular tumor or tumors may be recognized and these would be particularly important as cancer antigens.